While that is true, the more interesting question is why some people are driven to consume a large number of calories despite the desire to be leaner.
Also consider that even obese people tend to reach a balance in their calorie consumption and expenditure even without a change in diet or an increase in physical activity.
Let's phrase it more precisely then: Populations of human beings do not grow obese by consuming relatively high proportions of dietary fat. They grow obese by consuming relatively high proportions of starch and sugar.
If you chained a person to a bed and stuffed them with lard every day, you could probably make them obese. But that's not what we are talking about. People who are eating according to their appetite get fat from consuming too much sugar and starch, which screws up the whole endocrine system. They do not get thrown out of wack and end up consuming too much food when they feed on fat.
Do you have any journal articles conclusively proving this? I'm aware of the impact sugar has on insulin levels and the insulin -> fat path, but I believe the medical consensus is that weight gain is primarily determined by the amount of calories eaten, not the type.
You are correct. So far the consensus opinion in medical research is that except for extreme ends of the dietary spectrum a calorie is a calorie. If you are trying to lose weight then some diets make the task a little easier than others, but otherwise the only equation that really matters is calories in as food vs. calories out as physical exertion.
The quotations of leading nutritionists need to be plastered on discussions like this, as they invalidate most of the comments.
"Weight loss achieved in clinical trials of calorie restricted diets are so small as to be clinically insignificant." --Cochrane Collaboration 2002
"It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures. So far, data to support this hypothesis are not compelling." --AHA/ACSM 2007
"The classical theory that fat is deposited in the adipose tissue only when given in excess of the caloric requirements has finally been disproved." --Ernst Wertheimer, Physiological Reviews [i.e. people in caloric deficit can still be getting fatter.]
"It may be stated categorically that the storage of fat, and therefore the production and maintenance of obesity, cannot take place unless glucose is being metabolized. Since glucose cannot be used by most tissues without the presence of insulin, it also may be stated categorically that obesity is impossible without the presence of insulin. It also may be stated categorically that obesity is impossible in the absence of adequate tissue concentrations of insulin... Thus an abundant supply of carbohydrate food exerts a powerful influence in directing the stream of glucose metabolism into lipogenesis, whereas a relatively low carbohydrate intake tends to minimize the storage of fat." --Edgar Gordon, JAMA
There's no study where they locked some people in a house full of potatoes and some people in a house full of bacon and showed that the potato people got fat. But the weight of evidence that starch and sugar cause obesity is overwhelming. Watch the video.
The consensus of that video was that calories eaten vs. used/expelled is the primary factor behind weight gain, but that some types of foods contribute more to weight gain than others for various reasons.
For example, fats are more easily passed through the digestive system (in the sense that they often just flow through to the other side), while sugars and starches are more likely to remain in the system and actually get digested. (Incidentally, the body's inability to fully digest fats is the way Alli works -- it makes your body even less capable of digesting fats, so if you eat too much fat, it comes out your ass.)